1. Field of the Invention
The present invention relates to an extrusion-type coating apparatus in which a coating solution continuously extruded toward a surface of a moving substrate is applied to the substrate surface at high speed to provide a thin-layered coat of a uniform thickness.
2. Background
Various extrusion-type coating apparatuses have been proposed in which a coating solution is applied to a surface of a moving substrate at high speed to provide a thin-layered coat of a uniform thickness. Also, various methods have been proposed for preventing the entrapment of air in the coating layer. Included in these methods are a method of sealing an upstream edge of a coating head by means of a precoating layer, a method of precoating utilizing a solution having an identical composition as the coating solution as disclosed in Japanese Patent Application Laid-Open No. 205561/1983 and a method of applying a solvent serving as a precoat to a substrate as disclosed in Japanese Patent Application Laid-Open No. 139929/1986. It is possible to realize thin-layered coatings at high speed utilizing these coating methods. However, the problems with each of these methods are as follows. In particular, in the extrusion-type coating apparatus used in each of the above methods, a space between a downstream edge of the coating head and the substrate is set in a pressurized state and a coating layer is subjected to smoothing. As a result of this smoothing process, the thickness of the coating layer is often not uniform if the thickness of the substrate is not uniform. Furthermore, if a foreign substance is disposed on the substrate, in the precoating solution or in the coating solution of the coating layer, the foreign substance is liable to become trapped at the downstream edge of the coating head resulting in the formation of streaks in the final coating.
It should be noted that typical coating solutions referred to herein include, for instance, photographic light-sensitive coating solutions, magnetic coating solutions, surface-protecting/antistatic or lubricating coating solutions, and the like. Typical resulting products include various photographic films, printing papers, magnetic recording media such as magnetic disks and magnetic tapes, and the like.
The present applicant has filed a copending application directed to a coating apparatus disclosed in Japanese Patent Application Laid-Open No. 20069/1988. This coating apparatus is capable of overcoming the above problems concerning coating streaks and nonuniform coating thickness attributable to nonuniform substrate thickness and the Young's modulus. Additionally, this apparatus is capable of reducing the pressure loss during the passage of the coating solution through the slot of the coating head.
In more detail, referring to FIG. 5, Japanese Patent Application Laid-Open No. 20069/1988 discloses a coating apparatus in which a coating surface of a substrate is liquid-sealed by an organic solvent applied in advance thereto. Thereafter, a coating layer is formed on the substrate by means of a front edge of the coating head, located on the upstream side in the moving direction of the substrate, and a sharp back edge, located on the downstream side. As described below, the back edge is displaced further from the substrate than the front edge.
According to this apparatus, as illustrated in FIG. 5 an organic solvent 6 is first applied to the coating surface of the substrate by means of a conventional coating apparatus such as a gravure coater, a roll coater, a blade coater, and an extrusion coater. This solvent layer 6 prevents the entrained air disposed on the upstream side of the front edge of the coating head from entering into the coating layer 7, thereby permitting high-speed coating in which a defect-free coating state is maintained.
As noted above, the back edge 3 is disposed such that a corner portion thereof is located further from the substrate than a tangent to the rear portion of the front edge 2 defining the outlet portion of a slit 8, as illustrated in FIG. 5. As a result, since a pressurizing force due to a substrate 1 does not act on the back edge 3, it is possible to prevent foreign substances from being trapped at the corner portion of the back edge 3 so as to thereby prevent the substrate from being scraped by the back edge 3. Furthermore, even if the substrate surface is not flat due to wariness (i.e., depressions in the substrate), the thickness of the coating remains substantially uniform in spite of the nonuniform thickness of the substrate. As a result, an excellent coated product can be obtained.
With this coating apparatus, however, there has been a problem caused by disturbances, such as the fluttering motion of the substrate or pressure fluctuations of the solution being fed which occur in a substrate-transporting system or a solution-feeding system during the coating process. As a result of theses disturbances, the coating solution adheres to the inclined surface of the back edge on the downstream side thereof resulting in a smear which is slightly hardened near the corner portion. This smear damages the coating surface, resulting in coating defects such as streaks.
In addition, similar coating defects such as streaks occur due, not only to the smearing of the back edge, but also to the breaking off of the corner portion of the back edge.
An object of the present invention lies in providing a coating apparatus capable of overcoming the above-described conventional problems and of stably forming uniform thin-layered coating films free of streaks and the like at high speed.